A key unit is one kind of part constituting a mobile device such as a portable telephone in which a large number of switch operation keys (push buttons) are aggregated and arranged on one sheet face. One key is made up of a key top made of a hard resin or the like attached to a surface of a key pad made of a soft material such as a silicone rubber or a thermoplastic elastomer; and the key pad having a switch pressing projection (so-called “pressing member”) on its back face. The keys are interconnected by the key pad. By disposing a circuit board provided with switch elements on the lower face of the key unit constituted as described above, a key switch is formed at a position corresponding to each key. An illumination type key unit as one kind of key unit as described above has a construction in which the character, the symbol, or the like, of each key is irradiated by a light from a light source.
Because the key top is disposed at a position to be most conspicuous in the objective mobile device, a special attention is paid to its design and ornamentation. A key top in which a metallic film is formed on the whole surface of the key top or the bottom face, which is a face facing the key pad, and the top face, by plating or the like, (hereinafter referred to as “metallic key”) is welcome because it has both of durability and high-grade feeling.
Normally, on a surface of the metallic key, a character, a symbol, or the like, indicating the function of the key is formed. As a method for forming the character, the symbol, or the like, on the metallic key, marking processing by a laser is thinkable. However, in forming the character, the symbol, or the like, directly on the metallic key in which a metallic film having a thickness of 0.1 to 30 μm and high surface reflectance has been formed by plating or the like on its key top made of plastic, by laser marking, which completely removes the metallic film at the irradiated portion, there is a difficult point in comparison with simply marking or cutting a metallic plate or the like by laser.
For example, if a character, a symbol, or the like, is tried to be formed directly on a key top in which mirror plating with chromium or the like has been applied to a surface of a key made of plastic, using a near infrared light of a wavelength of 1064 nm as the fundamental wave of Nd:YAG (an yttrium aluminum garnet crystal doped with neodymium ions), which is a solid laser used widely, there is a problem in which the temperature of the portion other than the irradiated point rises before the objective character or the like is formed, and sufficient processing can not be performed because the underlayer plastic is melted or the like. It is thinkable that this is because the energy density at the irradiated point is insufficient because the spot diameter is difficult to be throttled by a lens optical system because the near infrared light is relatively long in wavelength.
Because of such a difficulty of laser marking to a metallic key, for manufacturing a partially plated key in which a character, a symbol, or the like, as an unplated portion, is mixed on a plated surface, the following complicated process was hitherto performed (for example, JP-A-2001-73154). That is, according to JP-A-2001-73154, a processing procedure is performed as “molding a plastic key top→surface roughness/activation→electroless plating with copper→electroless plating with tin on copper→laser marking processing on a tin plating layer (exposure of a copper plating layer)→removing the copper plating layer at a character portion by etching→electroless plating with nickel (other than the character portion)→electro plating with gold on nickel”.
In addition, there is a marking method called “Shibo process”. This is a method in which a plane aggregation of a large number of very small recessed points (recesses having its diameter and depth of 10 to 30 μm) is formed on the metallic film on the metallic key surface. For representing the shape of a character or the like by this Shibo process, conventionally, an electroformed mold is used in general. The electroformed mold is used by being incorporated in a mold for key top molding, and has a portion on which the plane aggregation of very small recessed points for forming a character or the like on the key top surface has been reversely transferred.
An example of an electroformed mold making process is as follows. First, a matrix for a key top having a non-marked surface is prepared with a synthetic resin, a copper alloy, or the like, and a desired pattern such as a character or a symbol is formed on the matrix by adequate surface roughness means. And, a film of a mold release agent is adhered to the matrix, and further, electro conductivity is given by applying silver mirror processing in case of the matrix of a synthetic resin, and then metallic plating is applied till the thickness reaches several millimeters. This plating process requires several tens days, and this process is called “electroforming”. After completion of electroforming, by peeling the portion formed by plating off the matrix, an electroformed mold can be obtained. And, a resin capable of plating, such as ABS (acrylonitrile-butadiene-styrene copolymer), is injected into a mold for molding a key top, in which the electroformed mold is incorporated, to form a key top. Next, by forming a metallic film on a surface of the key top by plating, a metallic key on which a pattern such as a character or a symbol has been formed by a Shibo process can be obtained. However, the electroformed mold has a defect that it can not rapidly cope with a change in pattern such as a character or a symbol.
Thus, like the case wherein a character, a symbol, or the like, is directly formed by laser marking, which completely removes the metallic film at the irradiated portion, it is thinkable that part of the metallic film, that is, only a surface layer portion of the metallic film, is removed by laser marking and a plane aggregation of a large number of very small recessed points is formed to perform a Shibo process. Actually, marking a character or the like by a Shibo process by laser on a iron plate or an aluminum plate. However, when the film thickness of metal is too thin, it is impossible to apply a Shibo process. In addition, even in case that the film thickness is relatively thick, there were many difficult problems in comparison with a case of performing to a simple metallic plate.